Zone selective interlocking (ZSI) systems have been available in the electrical power distribution industry for many years. The standard electrical power system protection provides selective-protective coordination between an upstream (main) breaker and the downstream (feeder) breakers. Thus, in the event of a fault (e.g. a short circuit, ground fault or an overload) the standard protection system selectively coordinates the upstream and downstream breakers so that the nearest downstream breaker will clear the fault before the upstream breaker opens. Therefore, because a downstream breaker nearest to the fault clears the fault a minimal number of feeders are de-energized.
Selective-protective coordination between upstream and downstream breakers is achieved by adding an additional time delay to the trip unit of the upstream breaker to thereby give the downstream breaker time to interrupt the fault. The ZSI system adds to the standard selective-protective coordinated system by allowing the upstream breaker to identify a fault within its zone (ahead of the feeder breakers) and clear this fault without adding the time delay required by selective-protection coordination. More specifically in a selectively coordinated protective system with ZSI, when a downstream breaker detects a current greater than its ground fault (GF) pick-up, short time (ST) pick-up or its instantaneous (I) pick-up it will send a restraint signal back to the upstream breaker. The upstream breaker, upon seeing the restraint signal, will begin to time out based on its normal selective-coordination GF or ST time-delay-trip setting. In a first scenario, if the downstream breaker operates properly it will trip thereby clearing the fault. Further, the upstream breaker will stop timing its GF or ST time-delay-trip setting and, thus, will not trip. In this first scenario, the downstream breaker cleared the fault and a minimal number of feeders were affected. In a second scenario, if the downstream breaker detects the fault and sends a restraint signal to the upstream breaker but the downstream breaker does not operate properly to clear the fault the GF or ST time-delay-trip setting on the upstream breaker will time out and the upstream breaker will trip thereby clearing the fault. Thus, the upstream breaker acts as a back up breaker to the downstream breaker in the event that the downstream breaker does not operate properly. In this second scenario, however, all feeders downstream from the tripped upstream breaker are de-energized. In a third scenario, if the upstream breaker with ZSI detects a GF or ST fault and does not receive a ZSI restraint signal from a downstream breaker, the upstream breaker will assume that the fault is in its protection zone (ahead of the feeder breakers) and will ignore its GF or ST time-delay-trip settings and will trip with minimal time delay thereby quickly clearing the fault. In this third scenario, if the ZSI signal connection between the downstream breaker and the upstream breaker were improperly connected, damaged or somehow malfunctioning, the upstream breaker would never receive a restraint signal and would always consider a fault to be within its zone and trip without a time delay on a GF or ST fault even if a downstream breaker was also in the process of clearing the fault. The improper or damaged ZSI signal connection, however, would not be detected until a fault occurred and the upstream breaker tripped with minimal delay thereby not only clearing the fault but also de-energizing all downstream feeders connected to the upstream breaker. In this scenario the selective-protective coordination of the power distribution system is totally lost. Thus, what is required is a ZSI system with a monitoring system to verify the ZSI signal interconnection between the upstream and downstream breakers that will: 1) override the non-coordination function of an upstream breaker in a ZSI system and 2) create an error message and alarm in the event of an improper or damaged ZSI signal connection between the upstream and downstream breakers.